This invention relates to a heat transfer sheet, and more particularly to a heat transfer sheet suitable for carrying out heat printing in accordance with image information by heating means such as thermal heads in order to form an image onto a heat transferable sheet or a sheet to be heat transferred. More specifically, this invention relates to a heat transfer sheet provided with a heat-resistant protective layer capable of preventing the fusion bonding between the heat transfer sheet and thermal heads in printing or running the thermal heads.
Heretofore, a heat sensitive color-producing paper has been primarily used in order to obtain an image in accordance with image information by means of thermal heads. In this heat sensitive color-producing paper, a colorless or pale-colored leuco dye (at room temperature) and a developer (such as bisphenol A) provided on a base paper are contacted and reacted by the application of heat to obtain a developed color image.
However, the heat sensitive color-producing paper as described above has serious drawbacks in that its color disappears and non-image areas form color when the resulting image is stored for a long period of time. Further, color printing is restricted to two colors, and thus it is impossible to obtain a color image having a continuous gradation.
On the other hand, products which have been recently used in order to overcome the drawbacks as described above are a heat sensitive fusing transfer sheet wherein a heat-fusing wax layer having a pigment or dye dispersed therein is provided on a sheet-shaped base; and a heat sensitive sublimation transfer sheet wherein a heat sensitive sublimation transfer layer comprising a dye having heat transferability and a binder therefor is provided on a sheet-shaped substrate.
When this heat sensitive fusing transfer sheet is laminated with a heat transferable sheet and then heat printing is carried out from the back of the heat sensitive fusing transfer sheet, the heat fusing wax layer containing the pigment or dye is transferred onto the heat transferable sheet to obtain an image. According to this printing process, an image having higher durability than that of the heat sensitive color-producing sheet can be obtained, and a multi-color image can be obtained by using a heat sensitive transfer sheet containing three primary color pigments or dyes and printing it many times.
On the other hand, in the heat sensitive sublimation transfer sheet, the binder layer containing the dye having heat transferability is provided on the sheet-shaped base or substrate. When this heat sensitive sublimation transfer sheet is laminated with a heat transferable sheet and then heat printing is carried out from the back of the heat sensitive sublimation transfer sheet, only dye present in the binder layer is heat sublimated and transferred on the heat transferable sheet to obtain an image. In this printing process, a multi-color image can be also obtained by using a heat sensitive sublimation transfer sheet containing three primary color dyes having heat transferability and printing it many times.
In recent years, there has been a growing demand for a method and means for obtaining an image having a continuous gradation like a color photograph directly from an electrical signal, and a variety of attempts have been made to meet this demand.
One of such attemps is a process for directly obtaining a silver salt color photograph from a cathode-ray tube (CRT) picture. However, this process is accompanied by the following drawbacks. The running cost is high. When silver salt film is a 35 mm film, the image cannot be instantly obtained because it is necessary to carry out a development treatment after the photographing.
An impact ribbon process and an ink jet process have been proposed as further processes. However, in these processes, the quality of the image is inferior and an image treatment is required. Thus, it is impossible to simply obtain an image like a photograph.
In order to overcome such drawbacks, an attempt has been made to carry out the recording by using the heat sensitive sublimation transfer sheet described above. In this process comprising using this heat sensitive sublimation transfer sheet, the dye having heat transferability present in the heat sensitive sublimation transfer layer is transferred onto the transferable sheet according to the amount of heat energy applied to the heat sensitive sublimation transfer sheet. Accordingly, an image having a continuous gradation can be obtained and recording can be carried out from a television signal by a simple treatment.
Examples of the bases of the heat transfer sheet heretofore used are condenser papers, polyester films, polypropylene films, cellophane and cellulose acetate films. The thickness of the base used is of the order of 10 microns.
Of these bases, if cost is regarded as being important, condenser paper has been used. If resistance to rupture during application processing, operating simplicity in a printer, uniform thickness and smooth surface are regarded as being important, plastic films have been used. Of plastic films, if the strength in the case of tissue paper is regarded as being important, a polyester film has been particularly preferably used.
A heat sensitive sublimation transfer layer comprising a heat sublimable dye and a binder therefor can be provided on such a polyester film, and heat printing can be carried out from the film surface provided with no heat sensitive sublimation transfer layer by means of a thermal head. However, when energy required for obtaining an image having a sufficient printing density is applied to the back surface of the film, the base sheet per se may fuse with the thermal head. Thus, so-called sticking phenomenon is observed and in some cases it is impossible to run the heat transfer sheet. In certain cases, the sheet may be broken from the fused portions.
In order to overcome these problems, several attempts have been proposed to provide the back surface of the base sheet of a heat sensitive fusing transfer sheet with a heat-resistant protective layer. Examples of such heat sensitive fusing transfer sheets are those wherein the back surface of a base is provided with a metallic layer or a silicone oxide layer as a wear-resistant layer (Japanese Patent Laid-Open Pub. No 143152/1979, and Japanese Patent Laid-Open Pub. No. 74195/1982), with a layer of heat-resistant resins such as silicone and epoxy resins (Japanese Patent Laid-Open Pub. No. 7467/1982), with a resin layer containing a surfactant which is solid or semisolid at room temperature (Japanese Patent Laid-Open Pub. No. 12978/1982), and with a pigment comprising a lubricating inorganic pigment and a heat-resistant resin therefor (Japanese Patent Laid-Open Pub. No. 155794/1981).
When the heat-resistant protective layer proposed in these Japanese Patent Laid-Open Publications is provided on the back surface of the heat sensitive sublimation transfer sheet to a film thickness of about 3 microns and then printing is carried out by means of a thermal head, in all cases, the sticking phenomenon is observed. Thus, the heat-resistant protective layers proposed cannot function as the protective layer.
This is because the heat-resistant protective layer of the heat sensitive sublimation transfer sheet described above undergoes high energy in printing. When heat energy required for heat sensitive fusing transfer recording is compared with heat energy for obtaining a sufficient recording density in a heat sensitive sublimation recording process wherein a sublimable dye is used, energy required for heat sensitive sublimation recording is at least about 1.5 times that required for heat sensitive fusing recording.
In order to overcome these problems, we have further carried out studies to find heat-resistant resins capable of using in the heat sensitive sublimation transfer sheet and to find systems wherein a lubricating material is incorporated in the resin described above. We have found the following facts.
In order to produce a heat sensitive sublimation transfer sheet provided with a heat-resistant protective layer by inexpensive processes such as coating rather than expensive processes such as vacuum deposition, it is necessary to use a resin having heat resistance as a base. In order to prevent the reduction of the heat sensitivity of a polyester film having a thickness of from about 6 to 10 microns, it is preferable that the heat-resistant protective layer has a thickness of from about 0.5 to 3 microns. In order to make it possible to carry out printing and running in the case of the heat transfer sheet provided with the heat-resistant protective layer having the thickness ranges described above, it is necessary to add any lubricating material to the resin base described above. When known inorganic materials such as talc and mica are added to the resin base as the lubricating material in a large amount to form a heat-resistant protective layer, running is not smooth and the solid areas become rough. Further, such inorganic materials may adhere to the thermal head.
In view of these findings, we have carried out further studies. As a result, we have now found that the use of a heat transfer sheet provided with a heat-resistant protective layer comprising specific components affords a heat transfer sheet capable of providing an image having a high density wherein the running of the thermal head is smooth during recording and the printing areas are not rough. The present invention has been developed on the basis of this discovery.